Measles is a constant threat not only in developing but also in developed countries. In the USA outbreaks in inner-cities of large metropolitan areas are perennial. In the face of major outbreaks the Immunization Practices Advisory Committee (ACIP) recommends that infants 6 months and older be given measles vaccine. In a study reported in FY89 it was found that the Edmonston-Zagreb measles vaccine grown in human diploid cells produced higher seroconversion rates than Schwarz vaccine when administered to children less than 1 year of age. This study was conducted in Mexico City in 1987. Previous studies conducted in children vaccinated against measles during the first year of life have suggested that these children lose antibody faster than children vaccinated after the first year of life and may have a poor response when revaccinated at an older age. We performed a follow-up study of children immunized in 1987. We evaluated antibody titers 20 months after vaccination in 1001 children who received one of three different doses (standard, medium or high) of Edmonston-Zagrab or Schwarz vaccine at 6 or 9 months of age. Twenty months after vaccination 95% of children in all vaccine groups had detectable plaque neutralizing measles antibody. The percentage of children with protective (> 200 m IU) antibody levels ranged from 80% in recipients of the standard dose Schwarz vaccine group to 95% in the medium and high dose Edmonston-Zagreb vaccine groups. Among children who had been vaccinated at 9 month, 95% and 96% of children who received standard dose Schwarz and Edmonston-Zagreb vaccines had antibody > 200 m IU, respectively. These data indicate that vaccination with Edmonston-Zabreb vaccine at 6 months of age results in persistence of protective antibody levels least 20 months post-vaccination.